[LoongArch] Transfer MI flags when expand PseudoCALL

When expanding a PseudoCALL, the corresponding flags (e.g. nomerge)
need to be passed to the new instruction.

This patch also adds test for the nomerge attribute.

The `nomerge` attribute was added during `LowerCall`, but was lost
during expand PseudoCALL. Now add it back.

Reviewed By: SixWeining

Differential Revision: https://reviews.llvm.org/D137888